The Best Models of Bodipy’s Electronic Excited State: Comparing Predictions from Various DFT Functionals with Measurements from Femtosecond Stimulated Raman Spectroscopy

Density functional theory (DFT) and time-dependent DFT (TD-DFT) are pivotal approaches for modeling electronically excited states of molecules. However, choosing a DFT exchange-correlation functional (XCF) among the myriad of alternatives is an overwhelming task that can affect the interpretation of results and lead to erroneous conclusions. The performance of these XCFs to describe the excited-state properties is often addressed by comparing them with high-level wave function methods or experimentally available vertical excitation energies; however, this is a limited analysis that relies on evaluation of a single point in the excited-state potential energy surface (PES). Different strategies have been proposed but are limited by the difficulty of experimentally accessing the electronic excited-state properties. In this work, we have tested the performance of 12 different XCFs and TD-DFT to describe the excited-state potential energy surface of Bodipy (2,6-diethyl-1,3,5,7-tetramethyl-8-phenyldipyrromethene difluoroborate). We compare those results with resonance Raman spectra collected by using femtosecond stimulated Raman spectroscopy (FSRS). By simultaneously fitting the absorption spectrum, fluorescence spectrum, and all of the resonance Raman excitation profiles within the independent mode displaced harmonic oscillator (IMDHO) formalism, we can describe the PES at the Franck–Condon (FC) region and determine the solvent and intramolecular reorganization energy after relaxation. This allows a direct comparison of the TD-DFT output with experimental observables. Our analysis reveals that using vertical absorption energies might not be a good criterion to determine the best XCF for a given molecular system and that FSRS opens up a new way to benchmark the excited-state performance of XCFs of fluorescent dyes

Multimode Charge-Transfer Dynamics of 4-(Dimethylamino)benzonitrile Probed with Ultraviolet Femtosecond Stimulated Raman Spectroscopy

4-(Dimethylamino)­benzonitrile (DMABN) has been one of the most studied photoinduced charge-transfer (CT) compounds for over 50 years, but due to the complexity of its excited electronic states and the importance of both intramolecular and solvent reorganization, the detailed microscopic mechanism of the CT is still debated. In this work, we have probed the ultrafast intramolecular CT process of DMABN in methanol using broad-band transient absorption spectroscopy from 280 to 620 nm and ultraviolet femtosecond stimulated Raman spectroscopy (FSRS) incorporating a 330 nm Raman pump pulse. Global analysis of the transient absorption kinetics revealed dynamics occurring with three distinct time constants: relaxation from the Franck–Condon L_a state to the lower locally excited (LE) L_b state in 0.3 ps, internal conversion in 2–2.4 ps that produces a vibrationally hot CT state, and vibrational relaxation within the CT state occurring in 6 ps. The 330 nm FSRS spectra established the dynamics along three vibrational coordinates: the ring-breathing stretch, ν_ph, at 764 cm^–1 in the CT state; the quinoidal CC stretch, ν_CC, at 1582 cm^–1 in the CT state; and the nitrile stretch, ν_CN, at 2096 cm^–1 in the CT state. FSRS spectra collected with a 400 nm Raman pump probed the dynamics of the 1174 cm^–1 CH bending vibration, δ_CH. Spectral shifts of each of these modes occur on the 2–20 ps time scale and were analyzed in terms of the vibrational anharmonicity of the CT state, calculated using density functional theory. The frequencies of the δ_CH and ν_CC modes upshift with a 6–7 ps time constant, consistent with their off-diagonal anharmonic coupling to other modes that act as receiving modes during the CT process and then cool in 6–7 ps. It was found that the spectral down-shifts of the δ_CH and ν_CN modes are inconsistent with vibrational anharmonicity and are instead due to changes in molecular structure and hydrogen bonding that occur as the molecule relaxes within the CT state potential energy surface

Stimulated Resonance Raman and Excited-State Dynamics in an Excitonically Coupled Bodipy Dimer: A Test for TD-DFT and the Polarizable Continuum Model

Bodipy is one of the most versatile and studied functional dyes due to its myriad applications and tunable spectral properties. One of the strategies to adjust their properties is the formation of Bodipy dimers and oligomers whose properties differ significantly from the corresponding monomer. Recently, we have developed a novel strategy for synthesizing α,α-ethylene-bridged Bodipy dimers; however, their excited-state dynamics was heretofore unknown. This work presents the ultrafast excited-state dynamics of a novel α,α-ethylene-bridge Bodipy dimer and its monomeric parent. The dimer’s steady-state absorption and fluorescence suggest a Coulombic interaction between the monomeric units’ transition dipole moments (TDMs), forming what is often termed a “J-dimer”. The excited-state properties of the dimer were studied using molecular excitonic theory and time-dependent density functional theory (TD-DFT). We chose the M06 exchange–correlation functional (XCF) based on its ability to reproduce the experimental oscillator strength and resonance Raman spectra. Ultrafast laser spectroscopy reveals symmetry-breaking charge separation (SB-CS) in the dimer in polar solvents and the subsequent population of the charge-separated ion-pair state. The charge separation rate falls into the normal regime, while the charge recombination is in the inverted regime. Conversely, in nonpolar solvents, the charge separation is thermodynamically not feasible. In contrast, the monomer’s excited-state dynamics shows no dependence on the solvent polarity. Furthermore, we found no evidence of significant structural rearrangement upon photoexcitation, regardless of the deactivation pathway. After an extensive analysis of the electronic transitions, we concluded that the solvent fluctuations in the local environment around the dimer create an asymmetry that drives and stabilizes the charge separation. This work sheds light on the charge-transfer process in this new set of molecular systems and how excited-state dynamics can be modeled by combining the experiment and theory

Intermolecular Charge Separation in Aggregated Rhodamine Dyes Used in Solar Hydrogen Production

Various modern solar light-harvesting systems, including those used in photovoltaics and solar fuel production, depend on efficient electron transfer from a surface-bound molecular dye to nanoscopic semiconductor particles. However, the productive electron transfer competes with a variety of other relaxation pathways for the dye, and the dominant pathway can change dramatically depending on its environment. A new sulfur-substituted thiorhodamine dye was synthesized having exceptional light-harvesting qualities for solar energy applications and for solar hydrogen production in particular. The dye was created with a thiophene spacer bearing a phosphonate-ester (<b>1-Ester</b>) or phosphonic-acid (<b>1-Acid</b>) allowing for excellent solubility in MeCN or the ability to functionalize metal oxide semiconductor nanoparticles such as TiO₂. While <b>1-Ester</b> is found to be fully monomeric in MeCN, <b>1-Acid</b> readily forms H-aggregated dimers which, upon photoexcitation, undergo charge separation to an ion pair (IP) in 1.5 ps. For <b>1-Acid</b> dimers, the stabilization of the IP causes an increase in lifetime to 270 ps compared to the 75 ps lifetime of the monomer. When <b>1-Acid</b> is attached to TiO₂, the inhomogeneous surface creates a distribution of chromophore packing structures where a range of transition dipole coupling environments is present such that both excimers and IPs can form. In a variety of solvent environments, ultrafast electron injection was found to occur in <300 fs from the dye to the semiconductor while IP formation occurs in 2–4 ps. For all aggregates studied, the photophysics was the same whether pumped at 620 nm, exciting to the 0–0 absorption band, or at 565 nm to the 0–1 transition that is dramatically enhanced by transition-dipole coupling in the H-aggregate. Surprisingly, the long-time, >2 ns, persistent formation of the charge-separated state following charge injection to TiO₂ only accounts for ∼10% of the photoexcited population, with the dominant relaxation pathways being IP and excimer formation. IP and excimer formation lower the total energy of the aggregate below the conduction band edge of TiO₂, deactivating the electron transfer process. The implications of IP and excimer formation in systems for solar light harvesting are discussed

Rhodamine-Platinum Diimine Dithiolate Complex Dyads as Efficient and Robust Photosensitizers for Light-Driven Aqueous Proton Reduction to Hydrogen

Three new dyads consisting of a rhodamine (RDM) dye linked covalently to a Pt diimine dithiolate (PtN₂S₂) charge transfer complex were synthesized and used as photosensitizers for the generation of H₂ from aqueous protons. The three dyads differ only in the substituents on the rhodamine amino groups, and are denoted as <b>Pt-RDM1</b>, <b>Pt-RDM2</b>, and <b>Pt-RDM3</b>. In acetonitrile, the three dyads show a strong absorption in the visible region corresponding to the rhodamine π–π* absorption as well as a mixed metal-dithiolate-to-diimine charge transfer band characteristic of PtN₂S₂ complexes. The shift of the rhodamine π–π* absorption maxima in going from <b>Pt-RDM1</b> to <b>Pt-RDM3</b> correlates well with the HOMO–LUMO energy gap measured in electrochemical experiments. Under white light irradiation, the dyads display both high and robust activity for H₂ generation when attached to platinized TiO₂ nanoparticles (Pt-TiO₂). After 40 h of irradiation, systems containing <b>Pt-RDM1</b>, <b>Pt-RDM2</b>, and <b>Pt-RDM3</b> exhibit turnover numbers (TONs) of 33600, 42800, and 70700, respectively. Ultrafast transient absorption spectroscopy reveals that energy transfer from the rhodamine ¹π–π* state to the singlet charge transfer (¹CT) state of the PtN₂S₂ chromophore occurs within 1 ps for all three dyads. Another fast charge transfer process from the rhodamine ¹π–π* state to a charge separated (CS) RDM^(0•)-Pt^(+•) state is also observed. Differences in the relative activity of systems using the RDM-PtN₂S₂ dyads for H₂ generation correlate well with the relative energies of the CS state and the PtN₂S₂ ³CT state used for H₂ production. These findings show how one can finely tune the excited state energy levels to direct excited state population to the photochemically productive states, and highlight the importance of judicious design of a photosensitizer dyad for light absorption and photoinduced electron transfer for the photogeneration of H₂ from aqueous protons

Chromophoric Dyads for the Light-Driven Generation of Hydrogen: Investigation of Factors in the Design of Multicomponent Photosensitizers for Proton Reduction

Two new dyads have been synthesized and studied as photosensitizers for the light-driven generation of H₂ from aqueous protons. One of the dyads, <b>Dy-1</b>, consists of a strongly absorbing Bodipy (dipyrromethene-BF₂) dye and a platinum diimine benzenedithiolate (bdt) charge transfer (CT) chromophore, denoted as PtN₂S₂. The two components are connected through an amide linkage on the bdt side of the PtN₂S₂ complex. The second dyad, <b>Dy-2</b>, contains a diketopyrrolopyrrole dye that is linked directly to the acetylide ligands of a Pt diimine bis­(arylacetylide) CT chromophore. The two dyads, as well as the Pt diimine bis­(arylacetylide) CT chromophore, were attached to platinized TiO₂ via phosphonate groups on the diimine through sonication of the corresponding esters, and each system was examined for photosensitizer effectiveness in photochemical generation of H₂ from aqueous protons and electrons supplied by ascorbic acid. Of the three photosensitizers, <b>Dy-1</b> is the most active under 530 nm radiation with an initial turnover frequency of 260 h^–1 and a total of 6770 turnovers over 60 h of irradiation. When a “white” LED light source is used, samples with <b>Dy-2</b> and the Pt diimine bis­(arylacetylide) chromophore, while not as effective as <b>Dy-1</b>, perform relatively better. A key conclusion is that the presence of a strongly absorbing organic dye increases dyad photosensitizer effectiveness only if the energy of the CT excited state lies below that of the organic dye’s lowest excited state; if not, the organic dye does not improve the effectiveness of the CT chromophore for promoting electron transfer and the light-driven generation of H₂. The nature of the spacer between the organic dye and the charge transfer chromophore also plays a role in the effectiveness of using dyads to improve light-driven energy-storing reactions

Efficient Bimolecular Mechanism of Photochemical Hydrogen Production Using Halogenated Boron-Dipyrromethene (Bodipy) Dyes and a Bis(dimethylglyoxime) Cobalt(III) Complex

A series of Boron-dipyrromethene (Bodipy) dyes were used as photosensitizers for photochemical hydrogen production in conjunction with [Co^III(dmgH)₂pyCl] (where dmgH = dimethylglyoximate, py = pyridine) as the catalyst and triethanolamine (TEOA) as the sacrificial electron donor. The Bodipy dyes are fully characterized by electrochemistry, X-ray crystallography, quantum chemistry calculations, femtosecond transient absorption, and time-resolved fluorescence, as well as in long-term hydrogen production assays. Consistent with other recent reports, only systems containing halogenated chromophores were active for hydrogen production, as the long-lived triplet state is necessary for efficient bimolecular electron transfer. Here, it is shown that the photostability of the system improves with Bodipy dyes containing a mesityl group versus a phenyl group, which is attributed to increased electron donating character of the mesityl substituent. Unlike previous reports, the optimal ratio of chromophore to catalyst is established and shown to be 20:1, at which point this bimolecular dye/catalyst system performs 3–4 times better than similar chemically linked systems. We also show that the hydrogen production drops dramatically with excess catalyst concentration. The maximum turnover number of ∼700 (with respect to chromophore) is obtained under the following conditions: 1.0 × 10^–4 M [Co­(dmgH)₂pyCl], 5.0 × 10^–6 M Bodipy dye with iodine and mesityl substituents, 1:1 v:v (10% aqueous TEOA):MeCN (adjusted to pH 7), and irradiation by light with λ > 410 nm for 30 h. This system, containing discrete chromophore and catalyst, is more active than similar linked Bodipy–Co­(dmg)₂ dyads recently published, which, in conjunction with our other measurements, suggests that the nominal dyads actually function bimolecularly

From Seconds to Femtoseconds: Solar Hydrogen Production and Transient Absorption of Chalcogenorhodamine Dyes

A series of chalcogenorhodamine dyes with oxygen, sulfur, and selenium atoms in the xanthylium core was synthesized and used as chromophores for solar hydrogen production with a platinized TiO₂ catalyst. Solutions containing the selenorhodamine dye generate more hydrogen [181 turnover numbers (TONs) with respect to chromophore] than its sulfur (30 TONs) and oxygen (20 TONs) counterparts. This differs from previous work incorporating these dyes into dye-sensitized solar cells (DSSCs), where the oxygen- and selenium-containing species perform similarly. Ultrafast transient absorption spectroscopy revealed an ultrafast electron transfer under conditions for dye-sensitized solar cells and a slower electron transfer under conditions for hydrogen production, making the chromophore’s triplet yield an important parameter. The selenium-containing species is the only dye for which triplet state population is significant, which explains its superior activity in hydrogen evolution. The discrepancy in rates of electron transfer appears to be caused by the presence or absence of aggregation in the system, altering the coupling between the dye and TiO₂. This finding demonstrates the importance of understanding the differences between, as well as the effects of the conditions for DSSCs and solar hydrogen production

Deactivating Unproductive Pathways in Multichromophoric Sensitizers

The effects of solvent and substituents on a multichromophoric complex containing a boron-dipyrromethene (Bodipy) chromophore and Pt­(bpy)­(bdt) (bpy = 2,2′-bipyridine, bdt =1,2-benzenedithiolate) were studied using steady-state absorption, emission, and ultrafast transient absorption spectroscopy. When the Bodipy molecule is connected to either the bpy or bdt in acetonitrile, excitation ultimately leads to the dyad undergoing triplet energy transfer (TEnT) from the redox-active Pt triplet mixed−metal-ligand−to−ligand′ charge transfer (³MMLL′CT) state to the Bodipy ³ππ* state in 8 and 160 ps, respectively. This is disadvantageous for solar energy applications. Here, we investigate two methods to lower the energy of the ³MMLL′CT state, thereby making TEnT unfavorable. By switching to a low dielectric constant solvent, we are able to extend the lifetime of the ³MMLL′CT state to over 1 ns, the time frame of our experiment. Additionally, electron-withdrawing groups, such as carboxylate and phosphonate esters, on the bpy lower the energy of the ³MMLL′CT state such that the photoexcited dyad remains in that state and avoids TEnT to the Bodipy ³ππ* state. It is also shown that a single methylene spacer between the bpy and phosphonate ester is sufficient to eliminate this effect, raising the energy of the ³MMLL′CT state and inducing relaxation to the ³ππ*